If you select multiple tracks (via Shift- or Command-clicking) in your iTunes library, the summation info (total number of tracks, total duration, and total size) is displayed at the bottom of the main window.

It used to be that I would have to move a group of songs into a new playlist to see, for example, if I had room for them on my iPhone, or if I could finish listening to an album by a certain time or whatever. This new feature so handy!